Proto-oncogene c-kit encodes the transmembrane type m tyrosine kinase, KIT protein, which is the receptor for stem cell factor (SCF). KIT is structurally characterized by an extracellular (EC) domain with 5 immunoglobulin like repeats, a single transmembrane domain, a juxtamembrane domain (“JM”) and a cytoplasmic tyrosine kinase domain. The kinase domain consists of the N-terminal (TK1) and C-terminal (TK2) lobes that are separated by a hydrophilic kinase insert. The TK2 domain contains the kinase activation loop (“AL”), a critical hinged region of the kinase that must assume a particular conformation in order to allow full kinase activation.
The single letter amino acid sequence of wild type human KIT protein is shown in FIG. 8 (SEQ ID NO:2) (NP—000213). The nucleic acid sequence of KIT is encoded by nucleotides 1-2928 of the sequence shown in FIG. 9 (SEQ ID NO: 1).
Under normal circumstances SCF binds to KIT inducing homodimerization of the receptor leading to intrinsic kinase activity and resulting in autophosphorylation of tyrosine residues. KIT then becomes the docking site for various SH2 domain signaling molecules. The KIT receptor is expressed on melanocytes, mast cells, primitive hematopoietic cells, primordial germ cells, intraepithelial lymphocytes and interstitial cells of Cajal.
Imatinib mesylate (also known as STI-571) is a potent KIT tyrosine kinase inhibitor and is now standard of care in advanced gastrointestinal stromal tumors (“GIST”), targeting autoactivating imatinib-sensitive KIT mutations that are mainly located in the JM domain in this protein. As used herein the term “imatinib” is used to refer to imatinib mesylate or STI-571. Although imatinib is a potent inhibitor of the kinase activity of wild type KIT and certain JM mutant KIT isoforms, many mutant KIT isoforms are resistant to clinically achievable doses of imatinib. Imatinib can only bind to the inactive or “closed” conformation of KIT. However, KIT AL mutations not only activate kinase activity but also stabilize the activation loop in an “open” conformation that does not allow productive imatinib binding. Activating KIT AL mutations are found in association with AML, mast cell disease, in particular systemic mastocytosis (“SM”), a subset of sinonasal NK/T-cell and Non-Hodgkin Lymphoma, seminoma/dysgerminoma and imatinib-resistant GIST. (See, e.g., US 2004/0253205; US 2005/0054617 and references cited therein).
Mastocytosises are a very heterogeneous group of disorders characterized by an abnormal accumulation of mast cells in different tissues, mainly in the skin and the bone marrow, but also in spleen, liver, lymph nodes, and the gastrointestinal tract, depending on the nature of the disease, and can be found isolated or sometimes associated with other hematological malignancies in humans. Alterations of the KIT gene have been described in a significant proportion of the patients. Particularly interesting are acquired mutations resulting in a constitutively activated receptor, possibly involved in the increased number of mast cells in tissues. Because of the extreme heterogeneity of mast cell neoplasms, the diseases have been classified into different categories of mastocytosis (See, Metcalfe, J Invest Dermatol. 96:2S-4S (1991)). Mast cells are implicated in tumoral pathologies, particularly in systemic mastocytosises that are hematological diseases similar to myeloproliferative syndromes. Mutant KIT kinase can also be found in mastocytosis associated with other malignant hemopathies, or less frequently in isolated hemopathies such as acute myeloid leukemia and myeloproliferative or myelodysplastic syndromes.
Gain-of-function point mutations of the KIT AL are associated with certain human neoplasms, including systemic mast cell disorders, AML, seminoma and GIST (both primary and imatinib-resistant GIST). In the case of mast cell disorders, seminoma, and AML, the most commonly-associated KIT mutation is the replacement of the normal aspartic acid residue at codon 816 of the activation loop with a valine residue (D816V) (Akin, C. and D. D. Melcalfe, Ann Rev Med 55 (2004) 419-32; Longley, B. J. and D. D. Metcalfe, Hematology-Oncology Clinics of North America 13. (2000) 697-701; Metcalfe, D. D. and C. Akin, Leukemia Res 25 (2001) 577-82; Tefferi, A. and A. Pardanani, Curr Opin Hematol 11 (2004) 58-64; Valent, P. et al., Hematol. Oncol Clin North Am 17 (2003) 1227-41). The D816V mutation results in constitutive activation of KIT kinase activity and is predicted to help stabilize the AL in the active conformation. In addition to D816V, other mutations involving codon 816 have been reported in systemic mast cell disorders (D816Y, D816F), AML (D816Y) and/or seminomas (D816Y, D816H). Consistent with the structural model of imatinib binding to KIT, the kinase activity of all of these mutants is resistant to imatinib.
In view of the imatinib resistance observed in certain cancers with cells containing certain KIT mutant isoforms, there is a need for diagnostic and therapeutic procedures and compositions tailored to address this condition. Particularly there is a need for a treatment for cancer, mastocytosis and related disorders involving mutant KIT kinase. The invention provided herein satisfies this need.